Production of an artificial turf fiber with a non-circular cladding

ABSTRACT

A method for producing an artificial turf fiber, comprising:
         preparing a core polymer mixture from a core polymer and a thread polymer forming beads within the core polymer;   coextruding the core polymer mixture with a cladding polymer component into a monofilament, the core polymer mixture forming a cylindrical core, The cladding polymer component forming a cladding encompassing the core with a non-circular profile;   quenching the monofilament;   reheating the quenched monofilament;   stretching the reheated monofilament to deform the beads into threadlike regions; and   providing one or more of the stretched monofilaments as the artificial turf fiber.

FIELD OF THE INVENTION

The invention relates to the production of synthetic fibers, and morespecifically, of artificial turf fibers resembling grass blades. Theinvention further relates to producing artificial turf, which is alsoreferred to as synthetic turf.

BACKGROUND AND RELATED ART

Artificial turfs are a class of polymer-based floor textiles whichimitate natural grass in its visual appearance and physical properties.They are normally manufactured from synthetic fibers which are fixed toa synthetic carpet background. The synthetic fibers imitate naturalgrass blades and are formed from one or more extruded monofilaments.Mono or bi-component monofilaments are known from the state of the artto be used as basic materials for the production of artificial turffibers.

High quality artificial turf fibers should give a faithful reproductionof the qualitative behavior (e.g. visual appearance, wetting behavior)of natural grass. An important demand in this respect is resilience,with the ability of the pile to recover from compression as it typicallyoccurs during use of the artificial turf, e.g. after being trodden byball game players. For this purpose, monocomponent artificial turffibers are manufactured from polymers such as polyamide which providesufficient mechanical stiffness and elasticity.

In addition, high quality artificial turf fibers should fulfill therequirement of providing a soft, elastic outer surface to gain a closerresemblance of natural grass blades and reduce the risk of injurieswhich may occur upon high velocity body contact with the artificial turffibers. This may be achieved by wrapping or coating a resilient corefiber as a semifinished part with a layer of an appropriate syntheticmaterial such as polyethylene. A more sophisticated manufacturingtechnology is coextrusion, where the materials for the core fiber andthe elastic cladding are joined together in a fluid phase. As highmechanical forces and harsh environmental conditions act upon anartificial turf, the coating of the bicomponent fiber may wear off orthe cohesion between core and cladding may get lost through delaminationor splicing. A coextrusion technology addressing this drawback has beenproposed in DE 10307174 A1, where a multilayer monofilament is disclosedwith a third polymer component interfacing the core and the cladding toincrease cohesion.

WO 2015/144223 A1 discloses a method of manufacturing artificial turf,the method comprising the steps of: creating a polymer mixture, whereinthe polymer mixture is at least a three-phase system, wherein thepolymer mixture comprises a first polymer, a second polymer, and acompatibilizer, wherein the first polymer and the second polymer areimmiscible, wherein the first polymer forms polymer beads surrounded bythe compatibilizer within the second polymer; extruding the polymermixture into a monofilament; quenching the monofilament; reheating themonofilament; stretching the reheated monofilament to deform the polymerbeads into threadlike regions and to form the monofilament into anartificial turf fiber; incorporating the artificial turf fiber into anartificial turf carpet.

The publication “Design and Characterization of a Bicomponent Melt-SpunFiber Optimized for Artificial Turf Applications” by R. Hufenus et al.,Macromol. Mater. Eng. 298: 653-663,https://doi.org/10.1002/mame.201200088, discloses results of acomparative study of bicomponent artificial turf fibers with differentcross sections, some of which comprising a cylindrical core surroundedby a cladding with a non-circular profile.

INVENTION SUMMARY

The invention provides for a method for producing artificial turf fiberswith improved biomimetic properties, as well as producing artificialturf from these artificial turf fibers.

In one aspect, the invention relates to an artificial turf fibercomprising at least one monofilament, each of the at least onemonofilament comprising a cylindrical core and a cladding, the corecomprising a core polymer and threadlike regions, which are formed by athread polymer and embedded in the core polymer, the claddingsurrounding the core and having a non-circular profile and being formedby a cladding polymer which is miscible with the core polymer.

In general, the core-cladding structure may have the advantage that thecore may be optimized to provide properties, such as a certain degree ofelasticity or rigidity, which are desirable for each blade of artificialturf as a whole, while the cladding can be designed with specificsurface properties such as softness and visual appearance. Particularly,the core may comprise a core polymer and/or a thread polymer whichprovides sufficient rigidity to the artificial turf fiber that thedesired resilience of artificial turf blades manufactured from theseartificial turf fibers are achieved. For the particular case that a softcladding polymer is selected and the core polymer is the same polymer asthe cladding polymer, the resilience of the artificial turf fiber arisesfrom the threadlike regions alone and the thread polymer should bechosen accordingly.

The miscibility of the core polymer and the cladding polymer may renderadditional interfacing materials for providing a sufficient amount ofcohesion between core and cladding unnecessary. During manufacturingfrom a fluid state, the core polymer and the cladding polymer may mixwith each other, forming a quasi-monolithic transition zone between coreand cladding which provides a mechanical stability which is comparableto monocomponent fibers.

The non-circular profile of the cladding may increase thesurface-to-mass ratio for each artificial turf fiber compared to purelycircular-cylindrical fibers if a suitable non-circular geometry isselected. An artificial turf manufactured from these artificial turffibers may thus feature an improved coverage per unit area, which wouldconventionally be achieved by manufacturing the artificial turf with ahigher blade density. According to embodiments of the invention, theimproved coverage can be achieved with lower polymer consumption, whichmay result in reduced manufacturing costs. According to embodiments, thefinished bicomponent artificial turf fiber has a yarn weight between1200 and 2300 dtex.

Each monofilament is a cylindrical polymer fiber, where the term“cylindrical” denotes a general right cylinder, i.e. having its primaryaxis oriented perpendicular to its base plane or cross section.Specifically, each fiber produced can be a non-circular cylinder, i.e.having a non-circular cross section. Examples of a non-circular crosssection include an ellipse or a polygon. It is understood that the crosssections of core and cladding may be selected independently from eachother, and that each of the core and the cladding may have anon-circular cross section. In a non-limiting example, an ellipticalcore is surrounded by a bean-shaped cladding. In another non-limitingexample, the fiber has a circular core and a cladding with twoprotrusions extending away from the core with a length of at least thecore diameter.

According to embodiments, the profile of at least one of the protrusionscomprises an undulated section spanning at least 60% of one side of saidat least one protrusion. An undulated section is understood here as apart of the fiber profile which comprises a repetitive element that issmall compared to overall dimensions of the fiber. For the scope of thepresent invention, this is considered to be the case if at least twoinstances (i.e. one repetition) of the repetitive element fit on each ofthe at least one undulated protrusion, and its amplitude, for each ofthe at least one undulated protrusion, is not more than 25 percent of amaximum thickness of said protrusion.

Undulation may increase the surface-to-mass ratio further and thereforecontribute to the benefits mentioned above. Another advantageous effectmay be an increase in diffuse light scattering of artificial turfproduced from artificial turf fibers with the undulated profile comparedto fibers having a smooth surface. In addition, undulation may increaseresilience of the fiber. Undulation may also decrease adhesion ofliquids (e.g. rain water) to the fiber by providing guiding edges todroplets, i.e. undulation may increase fiber surface while decreasingliquid contact surface.

Artificial turf produced from artificial turf fibers with the undulatedprofile may therefore be produced more efficiently and have a shorterdrying time during usage.

According to embodiments, the undulated section spans one side of thenon-circular profile and the non-circular profile comprises no furtherundulated sections apart from the undulated side. In an example, thefiber is double-sided, comprising one smooth face (smooth side of theprofile, e.g. straight or concave) and one grooved face (undulatedside). In addition to the aforementioned general advantages ofundulation, a single-sided undulation may be a closer approach to bladestructures found with natural grass, which may contribute beneficiallyto the properties of an artificial turf manufactured with such fibers.In such artificial turf, a portion of the grooved face of each fiber maybe surfacing the turf in a stochastic distribution. This may give theturf a less homogeneous and matted appearance. In addition, using suchturf e.g. for athletic activities may locally give the artificial grassblades a defined orientation, such that the oriented contact areabecomes easily discernable from its stochastically oriented environment.

A “thread polymer” is understood here as any polymer which can be usedto form threadlike regions within the core of a stretched bicomponentmonofilament according to embodiments of the invention. The threadpolymer is preferably chosen to exhibit a high bending stiffness afterbeing stretched into threadlike regions as described herein. The bendingstiffness may be sufficiently high that no further means are needed toprovide a desired level of resilience to an artificial turf fibermanufactured from the monofilament. In solid form, the thread polymermay differ from the core polymer and/or the cladding polymer with regardto rigidity, polarity and/or density.

A “core polymer” may be any polymer which can be used to embed beads orthreadlike regions of a thread polymer to form the core of amonofilament according to embodiments of the invention. The core polymeris preferably not miscible with the thread polymer, but at leastpartially miscible with the cladding polymer. If an immisciblecombination of thread polymer and core polymer is chosen, the corepolymer is preferably selected such that the thread polymer can beembedded into the core polymer using a compatibilizer polymerinterfacing the thread polymer and the core polymer. Preferably, aninexpensive polymer is chosen as the core polymer as it is supposed toform the largest portion of the core by mass and/or volume.

The term “cladding polymer” is used here to refer to any polymer whichcan be used to surround a core strand formed by a core polymer and athread polymer to form a monofilament according to embodiments of theinvention. The cladding polymer should be miscible with the core polymerin fluid state. The cladding polymer is preferably chosen to exhibitsoft and smooth haptic properties as it is supposed to form the outerlayer, or cladding, of an artificial turf fiber according to embodimentsof the invention. Furthermore, a preferred cladding polymer is suitablefor coextrusion with a second component formed by a mixture of corepolymer and thread polymer. Preferably, the cladding polymer is aninexpensive polymer as it is supposed to form a major portion of thetotal mass or volume of a monofilament according to embodiments of theinvention.

The core of the artificial turf fiber, or the liquid core polymermixture from which the core is formed during manufacturing, is at leasta two-phase system comprising the thread polymer as a first one of theat least two phases, and the liquid core polymer as a second one of theat least two phases.

The thread polymer and the core polymer are two chemically differentpolymers. In any case, each of the core polymer and the thread polymerform a phase, i.e. a continuous volume filled with a plurality ofmolecules of the respective polymer. In this sense, any beads orthreadlike regions formed from the thread polymer are phases embedded inthe phase formed by the core polymer. More precisely, the term“threadlike region” is not to be understood as a single stretchedpolymer molecule, but rather as a continuous filament filled with aplurality of thread polymer molecules.

According to embodiments, the liquid core polymer mixture, orrespectively, the core, is a two-phase system, i.e., the core is free ofthe compatibilizer. A two-phase core may have the advantage that nocompatibilizer is needed for interfacing thread and core polymer if theyare incompatible. The thread polymer may form an embedded structurewithin the matrix of even an incompatible core polymer by mechanicallydispersing the thread polymer within the liquid core polymer.

Core polymer and cladding polymer may be different or identicalpolymers. According to embodiments, core polymer and cladding polymermay be different forms of the same polymer. More particularly, apreferred choice both for the core polymer and the cladding polymer ispolyethylene. In one embodiment, the core polymer is high-densitypolyethylene (HDPE) and the cladding polymer is linear low-densitypolyethylene (LLDPE). In liquefied form, this combination may feature ahigh miscibility with each other as well as rheological properties whichare optimized for forming a firm bond between core and cladding by meansof coextrusion. When formed into a monofilament for producing anartificial turf fiber according to embodiments of the invention, the twosolidified polymers may provide further advantages: HDPE is denser andmore rigid than LLDPE, which may thus add to the resilience of theartificial turf fiber, while LLDPE is soft and wear resistant, which mayprovide a reduced risk of injury and amended durability.

The invention recognizes that an artificial lawn may have advantageoustechnical and/or qualitative properties (such as its visual or hapticappearance or its behavior during sportive usage) if the fibers whichmake up its pile are equipped with biomimetic traits, i.e. if theyimitate the structural components and/or features of natural grass,particularly the outline of a normal cross section of a grass blade. Inan embodiment, the profile represents the cross section of a grass bladeof the genus Lolium.

According to embodiments, the cladding forms two protrusions whichextend from the core in opposite directions. The two protrusions of thecladding may give the artificial turf fiber a structure with a closerresemblance of blades of natural grass. This may result in a morenatural appearance as well as characteristics for the artificial turfwhich imitate the physical characteristics of a natural lawn duringusage more realistically.

According to embodiments, the profile of at least one of the protrusionscomprises a concave side. Compared to protrusions with straight sides,this may reduce the cross-sectional area of the fiber while slightlyincreasing its perimeter. Therefore, protrusions comprising a concaveside may increase the surface-to-mass ratio further, to the beneficialeffects described before. Preferably, the curvature of the concave sideis limited such that the thickness of the at least one concavely taperedprotrusion is smallest at the edge of the fiber, i.e. the protrusionsshould contain no “bottleneck” which might reduce mechanical stabilityof the fiber.

According to embodiments, the cladding is a hydrophobic polymer. Thismay yield a shorter drying time for a resulting artificial turf afterhumid weather conditions (e.g. rain or dew) or cleaning, which mayimprove its playability in turn.

According to embodiments, the cladding is joined to the core by acontact layer, and the contact layer consists of a mixture of the corepolymer and the cladding polymer. During production of the artificialturf fiber, a core polymer and the cladding polymer are heated to aliquid state. When these two miscible polymers come into contact, theywill mix with each other in an interfacing zone herein referred to as“contact layer”. When the monofilament precursor thus formed is cooleddown, the two polymers solidify so that the contact layer forms a solidconnection between both components which is void of any contact surface.The contact layer forms a three-dimensional structure which comprises agradual transition of polymer types. In some embodiments, the numberdensity of core polymer molecules gradually decreases from the coreoutward and the number density of cladding polymer molecules analogouslydecreases from the cladding inward. In the special case of identicalcore and cladding polymers, the number density of polymer moleculesremains constant, while only the concentration of additives which may bepresent in only one of the interfacing components forms a gradienttowards the respective other component.

Hence, core and cladding are connected by a substance-to-substance bondformed by a polymer mixture which is held together by intermolecularforces which may be stronger than purely adhesive forces acting acrosstwo different adjacent, but not intermixed polymers. The two polymersare bonded together in a way which is similar to the intermolecularforces present in a monocomponent fiber. Shear stress occurring duringuse of an artificial turf manufactured from such fibers will thereforebe less likely to delaminate the cladding from the core. An artificialturf according to embodiments of the invention may therefore feature animproved wear resistance.

A stronger connection between core and cladding may also contributebeneficially to the different means described herein for increasing thesurface-to-mass ratio of artificial turf fibers according to embodimentsand/or artificial turf according to embodiments. A fiber with anincreased surface or surface-to-mass ratio may be more susceptible toexternal forces, an effect which may increase the risk of delamination.However, the contact layer according to embodiments of the invention maycounterbalance this effect and therefore enable a larger increase insurface-to-mass ratio as would be possible without it.

Moreover, no compatibilizing polymer is needed to bring core andcladding into cohesive contact. Embodiments of the invention may achievean equal or stronger cohesion between core and cladding thanthree-component artificial turf fibers where the third component is acompatibilizer interfacing core and cladding. For this reason, theproduction of artificial turf fibers according to embodiments of theinvention may also result in a simplified production setup as only twocomponents must be brought into contact.

According to embodiments, the thread polymer is immiscible with the corepolymer, the core further comprises a compatibilizer surrounding each ofthe threadlike regions and interfacing the thread polymer and the corepolymer. The thread polymer, which is responsible for the resilience ofthe artificial turf fiber, may be selected from a range of materialswhich ensure a sufficient degree of stiffness with no regard tomiscibility with the core polymer. Therefore, a compatibilizer may beadvantageously used to stabilize the emulsion of the thread polymerbeads in the core polymer in fluid state during production.

The compatibilizer is a polymer of a specific microscopic structurewhich enables cohesion of the otherwise immiscible thread polymer andcore polymer. Instead, the desired resilience may be achieved by meansof the compatibilizer causing the threadlike regions, which can beformed from the stabilized beads by stretching the monofilament aftercoextrusion, to stay fixed in the core polymer matrix, forming anembedded structure.

According to embodiments, the compatibilizer is an amphiphilicsubstance. An amphiphilic substance is capable of connecting polar andnon-polar molecules. An amphiphilic compatibilizer may connect moleculesof e.g. a non-polar core polymer with molecules of a polar claddingpolymer, and vice versa.

According to embodiments, the core polymer is a non-polar polymer.

According to embodiments, the thread polymer is immiscible with thecladding polymer, the cladding is fixed to the core by a contact layer,the contact layer comprises a mixture of the core polymer and thecladding polymer, and the contact layer locally further comprises thecompatibilizer as a third component of the mixture.

During manufacturing of the artificial turf fiber, the threadlikeregions may be arranged at random radial positions of the core. Inparticular, it may happen that some of the threadlike regions arelocally or completely disposed at the boundary of the core. Threadlikeregions from the core boundary may therefore get introduced into thecontact layer during the described mixing process.

If the thread polymer is immiscible with the core polymer, thecompatibilizer surrounding the threadlike regions may likewise getintroduced into the contact layer. If the thread polymer is alsoimmiscible with the cladding polymer, the compatibilizer may have thebeneficial effect that the bonding force between core and cladding isnot diminished locally in areas where part of the threadlike regions,which cannot mix with the cladding, get introduced into the contactlayer. It may, however, be necessary to select a suitable compatibilizermaterial which is able to provide cohesion of the thread polymer withthe core polymer and with the cladding polymer as well.

According to embodiments, the thread polymer is a polar polymer.According to embodiments, the thread polymer is a hydrophilic polymer.

According to embodiments, the thread polymer is one of polyamide,polyethylene terephthalate, polybutylene terephthalate, polyester, andpolybutyrate adipate terephthalate, and/or the core polymer and/or thecladding polymer is any one of polyethylene, polypropylene, and amixture thereof.

Resilience of the artificial turf fiber may be achieved by using merelysmall portions of a thread polymer with a high bending resistance.Hence, a smaller amount of the mentioned polymers, which are comparablyexpensive, but may yield the desired level of bending stiffness, may beused compared to fibers where the core is manufactured from the threadpolymer as a whole. In contrast, the cladding and the largest portion ofthe core may be formed by the above-mentioned polymers, which are softand comparably inexpensive. This may provide for a soft and smoothartificial turf surface, which may be beneficial to reduce the risk ofinjuries upon high velocity skin contact during usage.

According to embodiments, first ones of the threadlike regions areformed by the thread polymer and second ones of the threadlike regionsare formed by an additional thread polymer, the additional threadpolymer being different from the thread polymer of the first threadlikeregions and being any one of the following: polyamide, polyethyleneterephthalate, polybutylene terephthalate, polyester, and polybutyrateadipate terephthalate. This may provide for a precise means ofcontrolling the size and distribution of the threadlike regions usingtwo different polymers.

According to embodiments, the additional thread polymer is a polarpolymer.

According to embodiments, the artificial turf fiber comprises:

-   -   the threadlike regions in an amount of 1 to 30 percent by weight        of the core, the threadlike regions comprising the thread        polymer and, optionally, an additional thread polymer; and/or    -   the compatibilizer in an amount of 0 to 60 percent by weight of        the core polymer mixture; and/or    -   the core polymer in an amount of 20-50 percent by weight of the        artificial turf fiber; and/or    -   the cladding polymer in an amount of 50-80 percent by weight of        the artificial turf fiber.

The mentioned percentage ranges may allow for choosing an optimalmaterial combination where, for instance, the requirements for fiberresilience, surface smoothness, and economic surface-to-mass ratio arebalanced.

According to embodiments, the core has a diameter of 50 to 600micrometers, the cladding has a minimum thickness of 25 to 300micrometers in all directions extending radially from the core, and eachof the protrusions has a radial extension, measured from the perimeterof the core, of the cladding thickness plus 2 to 10 times the radius ofthe core.

The mentioned ranges for the core diameter and the minimum claddingthickness may allow for an optimized dimensioning of the artificial turffiber to provide the desired degree of stiffness and a sufficient amountof cladding material surrounding the core to form the mechanicallyrobust contact layer. Said ratio of the radial extension of theprotrusions with respect to the core radius may be chosen so as toimprove the biomimetic properties of the artificial turf and thesurface-to-mass ratio of the artificial turf fibers.

According to embodiments, the threadlike regions have a diameter of lessthan 50 μm and/or a length less than 2 mm. A proper dimensioning of thethreadlike regions may allow for customizing the resilience of theartificial turf fiber to the expected usage conditions. If thethreadlike regions are manufactured with a too large diameter, anartificial turf manufactured with the artificial turf fibers might havean inappropriately hard or stiff surface. Another parameter is thelength of the threadlike regions: although the thread polymer may bechosen to provide a large bending stiffness compared to the otherpolymers present in the artificial turf fiber, they may become bendablewith a large bending radius if they are too long. In an optimizeddesign, the threadlike regions may be substantially shorter than anoverall length of an artificial turf blade and/or the full bendingcircle of a thread polymer cylinder of a given diameter, but still longenough that the low elasticity of the core polymer is not dominating.

According to embodiments, the core is free of at least one of thefollowing components of the cladding: a wax, a dulling agent, a UVstabilizer, a flame retardant, an anti-oxidant, a fungicide, a pigment,and combinations thereof. It may be beneficial to use one or more of thementioned additives only in the cladding where they are actually needed.This may allow for a more cost-effective production as less additivesare consumer per unit length of the artificial turf fiber.

According to embodiments, the at least one monofilament is a coextrusionproduct of a first coextrusion component and a second coextrusioncomponent, the first coextrusion component comprising at least the corepolymer and the thread polymer, the second coextrusion componentcomprising at least the cladding polymer. Forming the monofilaments byextrusion may allow for comparably inexpensive mass production of theartificial turf fibers. The use of coextrusion technology, i.e. bondingthe core and the cladding together while being in the fluid phase at thesame time, may yield a monofilament with improved protection againstdelamination or splicing due to shear stress and/or adverseenvironmental influences.

For example, the bicomponent artificial turf fiber can be manufacturedby coextruding the two polymer components through separate channels,e.g. an inner channel receiving the melted core polymer component and anouter channel receiving the melted cladding polymer component joiningthe components. Upon exiting the separate channels, the two componentswould be formed to a strand which is pressed through an extrusionopening.

In this scenario, the joining process is responsive to the flowcharacteristics downstream of the channels. Process parameters, mainlytemperature and feed rates, may be chosen such that a balance betweenlaminar flow and turbulent flow is achieved during joining. A purelylaminar flow could result in comparably weak adhesive bonding betweencore and cladding as the molecules from both components would not mixsignificantly. On the other hand, a pronouncedly turbulent flow couldcause instabilities which would destroy the core-cladding structure atleast locally. The process parameters were preferably balanced such thata small-scale turbulence would be created where the core and claddingmolecules could mix within a thin contact layer of nearly constant widtharound the core.

The contact layer constitutes a transition zone where the numberdensities of core polymer and cladding polymer molecules form agradient. This way, a bond strength between core and cladding may beobtained which surpasses bonding forces which can be achieved byadhesive bonding.

The strand thus formed of the joined components is then pressed throughan extrusion opening. The contour of the opening corresponds to theperimeter of the artificial turf fiber monofilament to be produced.Preferably, the extrusion opening comprises two circular or ellipsoidalsections which are located on two opposite sides from the center andwhich are connected to each other via two long, narrow protrusion gapslocated on two further opposite sides from the center. Hence, the centerof the joined strand pressed through the opening may comprise the coresurrounded by circular or ellipsoidal sections of the cladding, whilethe protrusion gaps would be filled by the cladding polymer componentonly. The described opening geometry may therefore yield a monofilamentwhich resembles a blade of natural grass more closely than e.g. acircular-cylindrical monofilament.

After exiting e.g. the coextrusion device, the monofilament may bequenched, e.g. by passing a water quench, and then annealed online,passing e.g. a heating oven and/or a set of heated godets. By thisprocedure the beads or droplets of the thread polymer, surrounded by thecompatibilizer, may be stretched into an axial direction of themonofilament and form small fiber-like, linear structures which may staycompletely embedded in the polymer matrix of the core polymer or locallymigrate into the contact layer.

Another aspect of the invention relates to an artificial turf comprisinga textile backing and multiple ones of the artificial turf fiberaccording to embodiments of the invention, the artificial turf fibersbeing incorporated into the artificial turf textile backing.

In some examples the stretched monofilament may be used directly as theartificial turf fiber. In other examples the artificial turf fiber maybe a bundle or group of several stretched monofilament fibers which maybe cabled, twisted, or bundled together. In some cases the bundle isrewound with a so called rewinding yarn, which keeps the yarn bundletogether and makes it ready for the later tufting or weaving process.

According to embodiments, the artificial turf fiber forms a pile on oneside of the artificial turf backing, wherein each of the artificial turffibers extends a predetermined length into the pile and the threadlikeregions have a length less than one half of the predetermined length.

According to embodiments, each of the monofilaments and/or theartificial turf fibers are fixed to the backing at a random radialorientation. A random orientation may yield an artificial turf withimproved pliability characteristics. As an example, the formation of aslippery surface by the artificial turf blades is more likely for anartificial turf where all the blades have the same radial orientation.Such artificial turf may therefore provide a higher grip upon treadingand, in addition, a more natural appearance.

In a further aspect, the invention relates to a method for producing anartificial turf fiber, wherein the method comprises:

-   -   preparing a liquid core polymer mixture, the core polymer        mixture comprising at least the thread polymer and the core        polymer, the thread polymer forming beads within the core        polymer;    -   coextruding the liquid core polymer mixture with a liquid        cladding polymer component into a monofilament, the liquid core        polymer mixture forming a cylindrical core, the liquid cladding        polymer component comprising the cladding polymer and forming a        cladding encompassing the core, the cladding having a        non-circular profile;    -   quenching the monofilament;    -   reheating the quenched monofilament;    -   stretching the reheated monofilament to deform the beads into        threadlike regions; and    -   providing one or more of the stretched monofilaments as an        artificial turf fiber.

The method comprises the step of preparing a core polymer mixture. Thecore polymer mixture as used herein encompasses a mixture of differenttypes of polymers and also possibly with various additives added to thecore polymer mixture. The term ‘polymer mixture’ may also be replacedwith the term ‘master batch’ or ‘compound batch’. The core polymermixture may be at least a three-phase system. A three-phase system asused herein encompasses a mixture that separates out into at least threedistinct phases. The core polymer mixture comprises a thread polymer, acore polymer, and a compatibilizer. These three items form the phases ofthe three-phase system. If there are additional polymers orcompatibilizers added to the system, then the three-phase system may beincreased to a four-, five-, or more-phase system. The thread polymerand the core polymer are immiscible. The thread polymer forms polymerbeads surrounded by the compatibilizer within the core polymer.

The method further comprises the step of coextruding the core polymermixture with a cladding polymer component into a monofilament. Toperform this extrusion the coextrusion components may for instance beheated. The method further comprises the step of quenching themonofilament. In this step the monofilament is cooled. The methodfurther comprises the step of reheating the monofilament. The methodfurther comprises the step of stretching the reheated filament to deformthe polymer beads into threadlike regions and to form the monofilamentinto an artificial turf fiber. In this step the monofilament isstretched. This causes the monofilament to become longer and in theprocess the polymer beads are stretched and elongated. Depending uponthe amount of stretching the polymer beads are elongated more.Stretching does not affect the cohesion between core and cladding as itdoes not introduce a differential speed between them.

The term ‘polymer bead’ or ‘beads’ may refer to a localized piece, suchas a droplet, of a polymer that is immiscible in the core polymer. Thepolymer beads may in some instances be round or spherical oroval-shaped, but they may also be irregularly-shaped. In some instancesthe polymer beads will typically have a size of approximately 0.1 to 3micrometer, preferably 1 to 2 micrometer in diameter. In other examplesthe polymer beads will be larger. They may for instance have a size witha diameter of a maximum of 50 micrometer.

The monofilaments formed by coextrusion of the core polymer mixture withthe cladding polymer component may already feature a robust bond betweencore and cladding. However, the coextruded monofilament is not yetresilient because the thread polymer is only present as bead within thecore polymer after quenching. The high elasticity offered by a rigidthread polymer may only be reached if the beads are extended intothreadlike regions whose elasticity follows the same principle as thatof a leaf spring. This extension may be achieved by reheating themonofilament and stretching it over a controlled length ratio. As aresult, an artificial turf fiber is formed which may feature a highresilience due to a highly elastic core, optimized surface propertiesdue to an appropriate choice of the cladding polymer, and inherentprotection from splicing or delamination due to a highly stable contactlayer where the core polymer is mixed with the cladding polymer.

Embodiments of the invention include forming the artificial turf fiberwith particular geometry features of the non-circular profile. This maybe done by pressing the bicomponent strand or precursor through anextrusion opening which has the non-circular profile, allowing theliquid cladding polymer component to fill the non-circular profile.According to embodiments, the coextruding further comprises forming thecladding with two protrusions which extend from the core in oppositedirections.

According to embodiments, the profile of at least one of the protrusionscomprises a concave side. According to embodiments, the profile of atleast one of the protrusions comprises an undulated section spanning atleast 60% of one side of said at least one protrusion. Possibleadvantages of the respective profile geometries are discussed furtherabove.

According to embodiments, the extrusion opening is located downstream ofa channel where the bicomponent polymer strand is allow to proceed in alaminar flow. This may improve the geometric stability of edges of theartificial turf fiber created by corners or narrow sections of thenon-circular profile.

According to embodiments, the coextruding further comprises bringing theliquid core polymer mixture and the liquid cladding polymer componentinto contact with each other such that a contact layer is formed betweenthe liquid core polymer mixture and the liquid cladding polymercomponent, the contact layer comprising a mixture of the liquid corepolymer mixture and the liquid cladding polymer component. This may beachieved by controlling the flow characteristics (streaming pattern,velocity distribution, viscosities, shear moduli, temperature, melt flowindices, etc.) during the joining such that a stable, small-scaleturbulence is created which causes the two components, which aresupposed to be distributed separately in the upstream, to percolate in athin region interfacing the core polymer mixture and the claddingpolymer component. Possible beneficial effects of the method accordingto said embodiments, including a strengthened cohesion between core andcladding of the finished artificial turf fiber, are discussed throughoutthe present description.

According to embodiments, the coextruding is performed such that theliquid core polymer mixture and the liquid cladding polymer componententer the joining path with unequal flow rates. This may have theadvantageous effect that the flow in the joining path is maintained at astable, small-scale turbulence. This may support the formation of thecontact layer with a constant thickness. Eventually, this may provide abicomponent polymer fiber with increased shear stability.

According to embodiments, the contacting comprises pressing the liquidcore polymer mixture and the liquid cladding polymer componentconcentrically along a joining path, the core polymer mixture and thecladding polymer component being allowed to mix along the joining pathto form the contact layer, the contact layer being formed within anaxial length of the joining path of 3 to 7 times the diameter of theliquid core polymer mixture at the upstream end of the joining path.According to embodiments, the diameter of the liquid core polymermixture at the upstream end of the joining path is between 0.5 and 1.5mm, preferably 1.25 mm.

The term “joining path”, which may also be called a “common polymerpath”, is understood herein as a part, element, section, region, or thelike, of a capillary or channel system of a coextrusion spinneretadapted for producing bicomponent fibers of the core-cladding(core-sheath, skin-core) type. The joining path comprises at least twoinlet openings and one outlet opening and can be defined as a region offree channel space between these openings where two liquid polymercomponents, when fed through the at least two inlet openings, areallowed to come into contact with each other with no barrier in between.The joining path is typically located at the downstream end of thespinneret and may be immediately followed by the extrusion opening.

This may allow for adjusting the length of the joining path to thespecific properties, such as viscosity, melt flow index or shearmodulus, of the polymer components to be brought into contact and to thespecific process parameters, like temperature or pressure, to providebeneficial rheological properties for establishing a firm bond betweencore and cladding of the bicomponent fiber. The flow in the joining pathshould be maintained at a stable, small-scale turbulence. If the lengthof the joining path is chosen too long, turbulence may get suppressed byfeedback of increased wall-polymer interaction. On the other hand, a tooshort joining path may destroy stability of the turbulence such that thecontact layer becomes variable e.g. in thickness and position. Abicomponent fiber produced with a too short joining region may show nobeneficial surface properties anymore which are supposed to arise from aclear distinction between core and cladding.

According to embodiments, the method further comprises forming the corewith a diameter of 50 to 600 micrometers, forming the cladding with aminimum thickness of 25 to 300 micrometers in all directions extendingradially from the core, and forming each of the protrusions with aradial extension in a range of 2 to 10 times the radius of the core. Asexplained further above, the mentioned ranges for the core diameter andthe minimum cladding thickness may be beneficial for providing thedesired degree of stiffness and a sufficient amount of cladding materialsurrounding the core to form the mechanically robust contact layer. Saidratio of the radial extension of the protrusions with respect to thecore radius may be chosen so as to improve the biomimetic properties ofthe artificial turf and the surface-to-mass ratio of the artificial turffibers.

According to embodiments, the method is performed such that the contactlayer assumes a radial thickness between 10 and 150 micrometers.According to embodiments, the method is performed such that the contactlayer assumes a radial thickness between 10 and 50 percent of theminimum thickness of the cladding in all directions extending radiallyfrom the core. A contact layer within the given dimensions may bebeneficial for providing a firm connection between core and cladding,while sparing sufficient volumes of core and cladding so that theirrespective desired functions, e.g. resilience of the core and softnessof the cladding, are not adversely affected.

According to embodiments, the method is performed such that thethreadlike regions assume a diameter of less than 50 μm and/or a lengthof less than 2 mm. As discussed further above, a proper dimensioning ofthe threadlike regions may allow for customizing the resilience of theartificial turf fiber to the expected usage conditions.

According to embodiments, the core polymer mixture is prepared free ofat least one of the following components of the cladding: a wax, adulling agent, a UV stabilizer, a flame retardant, an anti-oxidant, afungicide, a pigment, and combinations thereof. It may be beneficial touse one or more of the mentioned additives only in the cladding wherethey are actually needed. This may allow for a more cost-effectiveproduction as less additives are consumer per unit length of theartificial turf fiber.

According to embodiments, the core polymer is high-density polyethylene(HDPE) and the cladding polymer being linear low-density polyethylene(LLDPE). In liquefied form, this combination may feature a highmiscibility with each other as well as rheological properties which areoptimized for forming a firm bond between core and cladding by means ofcoextrusion. When formed into a monofilament for producing an artificialturf fiber according to embodiments of the invention, the two solidifiedpolymers may provide further advantages: HDPE is denser and more rigidthan LLDPE, which may thus add to the resilience of the artificial turffiber, while LLDPE is soft and wear resistant, which may provide areduced risk of injury and amended durability.

According to embodiments, the liquid core polymer mixture is at least athree-phase system, the core polymer mixture further comprises acompatibilizer, and the preparing of the liquid core polymer mixtureresults in the beads being surrounded by the compatibilizer and immersedin the core polymer. As explained in more detail further above, acompatibilizer may be advantageously used to stabilize the emulsion ofthe thread polymer beads in the core polymer in fluid state duringproduction.

It is understood that a person skilled in the art knows alternativeapproaches for preparing the core polymer mixture such that it comprisesbeads surrounded by a compatibilizer and immersed in the core polymer.In a non-exhaustive example, the compatibilizer is applied to agranulate of the thread polymer using a coating technique, andsubsequently the coated granules are added to the molten core polymer.

According to embodiments, the thread polymer is immiscible with thecladding polymer, the coextruding further comprising bringing the liquidcore polymer mixture and the liquid cladding polymer component intocontact with each other such that a contact layer is formed between theliquid core polymer mixture and the liquid cladding polymer component,the contact layer comprising a mixture of the core polymer and thecladding polymer, the contact layer locally further comprising thecompatibilizer as a third component of the mixture. This may have thebeneficial effect that the bonding force between core and cladding isnot diminished locally as an effect of lacking miscibility of claddingpolymer and thread polymer.

According to embodiments:

-   -   the core polymer mixture comprises the thread polymer and the        additional thread polymer combined in an amount of 1 to 30        percent by weight of the core polymer mixture; and/or    -   the core polymer mixture comprises the compatibilizer in an        amount of 0 to 60 percent by weight of the core polymer mixture;        and/or    -   the monofilament comprises the cladding polymer in an amount of        50-80 percent by weight of the monofilament.

The mentioned percentage ranges may allow for choosing an optimalmaterial combination where, for instance, the requirements for fiberresilience, surface smoothness, and economic surface-to-mass ratio arebalanced.

According to embodiments, the preparation of the liquid core polymermixture comprises:

-   -   forming a base polymer mixture by mixing the thread polymer with        the compatibilizer;    -   heating the base polymer mixture;    -   extruding the base polymer mixture;    -   granulating the extruded base polymer mixture;    -   mixing the granulated base polymer mixture with the core        polymer; and    -   heating the granulated base polymer mixture with the core        polymer to form the liquid core polymer mixture.

This particular method of preparing the polymer mixture is presentedhere as a first alternative and may be advantageous because it enablesvery precise control over how the thread polymer and compatibilizer aredistributed within the core polymer. For instance, the size or shape ofthe extruded base polymer mixture may determine the size of the polymerbeads in the core polymer mixture.

In the aforementioned method of preparing the core polymer mixture, forinstance, a so called one-screw extrusion method may be used. As analternative to this, the polymer mixture may also be created by puttingall of the components that make it up together at once. For instance,the thread polymer, the core polymer and the compatibilizer could be alladded together at the same time. Other ingredients such as additionalpolymers or other additives could also be put together at the same time.The amount of mixing of the core polymer mixture could then beincreased, for instance, by using a two-screw feed for the extrusion. Inthis case, the desired distribution of the polymer beads can be achievedby using a proper rate or amount of mixing.

It is understood that the step of heating the granulated base polymermixture with the core polymer concludes the preparation of the corepolymer mixture, which in turn is the first step of the method forproducing an artificial turf fiber according to embodiments of themethod disclosed herein. Hence, it is clear that the step of coextrudingthe core polymer mixture requires that the core polymer mixture isprepared beforehand, and that the core polymer mixture must be melted(see definition further below) in order to be able to perform thecoextrusion.

In a second alternative, it is possible to prepare the liquid corepolymer mixture by mixing solid granulates of the core polymer, thethread polymer and the compatibilizer, and subsequently melting them.This may likewise yield the core polymer mixture with beads of thethread polymer being surrounded by the compatibilizer and immersed inthe core polymer by statistical self-alignment of the liquidcompatibilizer into the energetically favorable state of interfacingdroplets of the liquid thread polymer and the surrounding matrix of theliquid core polymer.

In both alternatives, the step of heating results in melting the corepolymer mixture, so that a subsequent coextrusion with the claddingpolymer component is possible. In other words, the core polymer mixture,which can generally be regarded as being composed of a mixture ofmultiple phases, is defined as melted if the combined volume of allliquid phases (at a given temperature) is larger than the combinedvolume of its solid phases. Thus, the melted core polymer mixture can beeither liquid in all of its phases, or it may contain a minor portion ofdispersed solid phases with a melting point above the processtemperature which are extruded together with the major portion of liquidphases.

According to embodiments, the coextrusion is performed at workingtemperatures between 180 and 270° C. This may be a temperature rangewith beneficial rheologic properties for many polymers, such aspolyethylene and/or polyamide, which are typically used for theproduction of artificial turf fibers. Said temperature range may beparticularly beneficial for creating a stable, small-scale turbulence ina joining path where the core polymer mixture and the cladding polymercomponent are brought into contact with each other, thus causing thecore polymer mixture and the cladding polymer component to mix in a thincontact layer interfacing core and cladding. Said temperature range mayalso be beneficial for allowing the melted cladding polymer component tofill the whole non-circular profile of the coextruded artificial turffiber, including narrow regions and/or boundary areas with a high flowresistance, completely and uniformly without edge instabilities causedby undesirable turbulence.

The feed rate of the core polymer mixture and the cladding polymercomponent may be controlled independently from each other. Depending onthe viscosities and/or melt flow indexes of the two liquid polymersbrought into contact, the flow characteristics of the two polymers maybe controlled precisely by adjusting the flow velocity difference of thetwo polymers in the joining path. The flow may get turbulent if thevelocity difference exceeds a threshold which is characteristic for theparticular viscosities and/or melt flow indexes of the two interactingfluids. Feeding the core polymer mixture at a greater feed rate than thecladding polymer component may thus have the effect that the flow ismaintained at a stable, small-scale turbulence. This may result in theformation of a thin contact layer of constant thickness between core andcladding where the core polymer and the cladding polymer are intermixed.Eventually, the method may yield an artificial turf fiber with increasedshear stability.

According to embodiments, the core polymer mixture is at least athree-phase system, the thread polymer forming first ones of the beadswithin the core polymer, the core polymer mixture further comprising anadditional thread polymer, the additional thread polymer being differentfrom the thread polymer and being any one of the following: polyamide,polyethylene terephthalate, polybutylene terephthalate, polyester, andpolybutyrate adipate terephthalate, and the additional thread polymerforming second ones of the beads within the core polymer, the stretchingdeforming the first ones of the beads into first ones of the threadlikeregions and deforming the second ones of the beads into second ones ofthe threadlike regions. Manufacturing the threadlike regions from twodifferent polymers may provide a precise means for controlling the sizeand distribution of the threadlike regions.

As an alternative, the thread polymer could be used to make a granulatewith the compatibilizer separately from making the additional threadpolymer with the same or a different compatibilizer. The granulatescould then be mixed with the core polymer to make the core polymermixture. As another alternative to this, the core polymer mixture couldbe made by adding the thread polymer, the core polymer, the additionalthread polymer and the compatibilizer all together at the same time andthen mixing them more vigorously. For instance, an extruder could beused with a two-screw feed.

According to embodiments, the providing comprises forming the stretchedmonofilament into a yarn and/or weaving, spinning, twisting, rewinding,and/or bundling the stretched monofilament into the artificial turffiber. This may allow for producing an artificial turf where each of theartificial turf fibers is a monofilament or, alternatively, formed by aplurality of the monofilaments according to embodiments of theinvention. Producing artificial turf fibers from more than onemonofilament may beneficially provide a high-durability artificial turfwith a coarser and stiffer pile.

In yet another aspect, the invention relates to a method for producingan artificial turf, the method comprising:

-   -   generating an artificial turf fiber by performing the method for        producing an artificial turf fiber described herein,    -   incorporating the sections into an artificial turf backing, and    -   cutting the artificial turf fiber into sections, creating cut        surfaces which expose a contact layer between the core and the        cladding.

The method comprises the step of incorporating the artificial turf fiberinto an artificial turf backing. In some examples the artificial turfbacking is a textile or a textile matt. The incorporation of theartificial turf fiber into the artificial turf backing could for examplebe performed by tufting the artificial turf fiber into an artificialturf backing and binding the tufted artificial turf fibers to theartificial turf backing. For instance, the artificial turf fiber may beinserted with a needle into the backing and tufted the way a carpet maybe. If loops of the artificial turf fiber are formed then may be cutduring the same step.

The incorporation may comprise the step of binding the artificial turffibers to the artificial turf backing. In this step the artificial turffiber is bound or attached to the artificial turf backing. This may beperformed in a variety of ways such as gluing or coating the surface ofthe artificial turf backing to hold the artificial turf fiber inposition. This, for instance, may be done by coating a surface or aportion of the artificial turf backing with a material such as latex orpolyurethane.

The incorporation of the artificial turf fiber into the artificial turfbacking could for example be performed alternatively by weaving theartificial turf fiber into artificial turf backing (or fiber mat) duringmanufacture of the artificial turf carpet. This technique ofmanufacturing artificial turf is known from United States patentapplication US 2012/0125474 A1.

The method comprises the step of cutting the artificial turf fiber intosections. Each cut has a cross-section to the artificial turf fibersurface which is exposed to external influences such as wear, UVradiation, or reactive substances which may be dissolved, e.g. inrainwater. The usage of artificial turf fibers according to embodimentsof the invention for producing the artificial turf may result in anincreased resistance against such detrimental external influences. Thisin turn may yield an artificial turf with improved protection againstdelamination or splicing of the bicomponent artificial turf fibers.Another beneficial effect may be improved protection against loss ofresilience, as the threadlike regions exhibit only a small portion ofthe cut surface as a working surface for detrimental influences.

It is understood that one or more of the aforementioned embodiments ofthe invention may be combined as long as the combined embodiments arenot mutually exclusive.

SHORT DESCRIPTION OF THE FIGURES

In the following, embodiments of the invention are explained in greaterdetail, by way of example only, making reference to the drawings inwhich:

FIG. 1 shows a radial cross-section of a monofilament for producing anartificial turf fiber;

FIG. 2 visualizes the composition of a three-component core polymermixture;

FIG. 3 shows a schematic axial cross-section of a monofilament beforestretching;

FIG. 4 shows a schematic axial cross-section of a monofilament afterstretching;

FIG. 5 shows a monofilament, the cladding being transparent such thatthe contact layer between core and cladding becomes visible

FIG. 6 is a cross-sectional diagram of an artificial turf comprisingartificial turf fibers made from monofilaments;

FIG. 7 is a cross-sectional profile of an artificial turf fiber withprotrusions comprising an undulated and a straight section

FIG. 8 is a cross-sectional profile of an artificial turf fiber withprotrusions comprising an undulated and a concave section; and

FIG. 9 is a cross-sectional detail of a coextrusion device with ajoining path.

DETAILED DESCRIPTION

Like numbered elements in these figures are either equivalent elementsor perform the same function. Elements which have been discussedpreviously will not necessarily be discussed in later figures if thefunction is equivalent.

Bicomponent artificial turf fibers have each of their componentsdesigned to fulfill the opposing requirements of providing artificialgrass blades which are soft but resilient at the same time. While theresilience of an artificial turf fiber can be provided by selecting arigid material for the core strand, its cladding can provide a softsurface which is better fit to reduce the risk of injuries and imitatethe haptic and visual behavior of natural grass. However, no materialcombination of core and cladding polymers is known to date which meetsthese demands, but is also miscible in a liquid state duringmanufacturing such that the two materials can be laminated together. Forthis reason, the core and cladding of bicomponent artificial turf fibersare typically joined together by an interfacing layer of a third polymerwhich is cohesive to the two otherwise immiscible components. However,cohesive forces between the adjacent layers are not strong enough toprovide sufficient protection against splicing of the three layers.Against this background, the invention seeks to provide a bicomponentartificial turf fiber which is less prone to delamination and provide amore cost-effective surface-to-mass ratio as well as a closerresemblance of natural lawn.

FIG. 1 shows a schematic diagram of a cut through a monofilament 100according to embodiments of the invention, the cut being orientedperpendicularly with respect to the central axis of the monofilament100. It comprises a cylindrical core 110 and a non-circular cladding 102surrounding the core 110. The core 110 comprises a core polymer 112 andthreadlike regions which are embedded in the core polymer 112. Thethreadlike regions are formed from a thread polymer 202 which ispreferably a polymer with a high bending rigidity or stiffness such aspolyamide. The threadlike regions permeate the core polymer 112 in axialdirections and at random radial positions and/or orientations.

The core polymer 112 makes up the majority of the core volume and may beany polymer which is miscible with the cladding polymer forming thecladding 102. As the core polymer 112 makes up the largest portion ofthe core 110, it is preferably chosen to be a comparably inexpensivematerial such as polyethylene. The core polymer 112 may be immisciblewith the thread polymer 202. In this case, the threadlike regions aresurrounded by a compatibilizer 204, which is another polymer materialwith the capability to emulsify the thread polymer 202 with the liquidcore polymer 112. After manufacturing, the threadlike regions remaincohesively coupled to the core polymer 112 in the solidifiedmonofilament 100.

The core 110 may comprise 1 to 30 percent by its weight the threadpolymer 202 and, if any, an additional thread polymer combined.Particularly, the thread polymer 202 and, if any, the additional threadpolymer combined may be 1 to 20 percent by weight of the core 110. Moreparticularly, the core 110 may comprise 5 to 10 percent by its weightthe thread polymer 202 and, if any, the additional thread polymercombined. The core 110 may for instance have a diameter of 50 to 600micrometer in size. It may typically reach a yarn weight of 50 to 3000dtex.

The threadlike regions may have a diameter of less than 50 micrometers.Particularly, the threadlike regions may have a diameter of less than 10micrometers. More particularly, the threadlike regions may have adiameter between 1 and 3 micrometers.

The cladding 102 is formed by a cladding polymer which is chosen to bemiscible with the core polymer 112 in fluid state. The cladding polymermay be identical to the core polymer 112. The annular cylindrical zoneor area where the cladding polymer contacts the core polymer 112 is acontact layer 114 where both polymers are mixed with each other. Hence,the contact layer 114 may bond core 110 and cladding 102 together withstronger forces than the long-range forces which occur typically withinarrangements with a purely cohesive bonding.

The cladding 102 completely surrounds the core 110 with two circularsections on two opposite sides of the core 110 and two flat, thin, longprotrusions 104 on two other opposing sides of the core 110. Thecladding 102 is preferably formed by a polymer such as polyethylenewhich may provide a soft and smooth surface characteristic. The cladding102 may comprise additives which support its interfacing function to theenvironment and/or a user. Typical additives to the cladding 102 may be,for example, pigments providing a specific color, a dulling agent, a UVstabilizer, flame retardant materials such as aramid fibers orintumescent additives, an anti-oxidant, a fungicide, and/or waxesincreasing the softness of the cladding 102.

Providing the cladding 102 with additives may have the advantage thatthese can be left out from the core 110. This way, a smaller content ofexpensive additive material per mass unit is required. As an example, itis not necessary to add pigments to the core 110 because only thecladding 102 is visible from the outside. By way of a more specificexample, it may be beneficial to add a green pigment, a dying agent anda wax to the cladding 102 to gain a closer resemblance of natural grassblades.

The non-circular profile of the cladding 102 may be symmetric orirregular; polygonal, elliptic, lenticular, flat, pointed or elongated.Preferably, the cladding 102 resembles a blade of grass by encompassingthe circular-cylindrical core 110 with two convex segments extending intwo opposite directions from the geometric center of the monofilamentand two flat protrusions 104 extending in two further oppositedirections from the geometric center of the monofilament, the convexsegments and the flat protrusions 104 being alternatingly joined byconcave segments. The two flat protrusions 104 may also add to thebiomimetic properties of the monofilament 100 and may increase thesurface-to-mass ratio for each monofilament 100 and, accordingly, mayprovide an improved surface coverage for an artificial turf manufacturedfrom artificial turf fibers on the basis of such monofilaments 100.

A monofilament 100 as shown in FIG. 1, which can also be referred to asa filament, can be produced by feeding a core polymer mixture 200 and acladding polymer component into a fiber producing coextrusion line. Thetwo polymer melt components are prepared separate from each other andthen joined together in the coextrusion tool, i.e., a spinneret plate,forming the two melt flows into a filament which is quenched or cooledin a water spin bath, dried and stretched by passing rotating heatedgodets with different rotational speed and/or a heating oven.

The thread polymer 202 is prepared by first mixing it with thecompatibilizer 204. This may result in granular material which consistsof a two-phase system in which the thread polymer 202 is surrounded bythe compatibilizer 204.

Then, a three-phase system is formed by adding the core polymer 112 tothe mixture whereby in this example the quantity of the core polymer 112is about 80-90 mass percent of the three-phase system, the quantities ofthe thread polymer 202 being 5% to 10% by mass and of the compatibilizer204 being 5% to 10% by mass. Using extrusion technology results in amixture of droplets or of beads 210 of the thread polymer 202 surroundedby the compatibilizer 204 that is dispersed in the polymer matrix of thecore polymer 112. In a practical implementation a so called master batchincluding granulate of the thread polymer 202 and the compatibilizer 204is formed. The master batch may also be referred to as a “polymermixture” herein. The granulate mix is melted and a mixture of the threadpolymer 202 and the compatibilizer 204 is formed by extrusion. Theresulting strands are crushed into granulate. The resultant granulateand granulate of the core polymer 112 are then used as the core polymermixture 200 in the coextrusion process described below.

FIG. 2 shows a diagram which illustrates a cross-section of a corepolymer mixture 200. The polymer mixture comprises a thread polymer 202,a core polymer 112 and a compatibilizer 204. The thread polymer 202 andthe core polymer 112 are immiscible. The thread polymer 202 is lessabundant than the core polymer 112. The thread polymer 202 is shown asbeing surrounded by compatibilizer 204 and being dispersed within thecore polymer 112. The thread polymer 202 surrounded by thecompatibilizer 204 forms a number of polymer beads 210. The polymerbeads 210 may be spherical or oval in shape or they may also beirregularly-shaped depending on how well the polymer mixture is mixedand the temperature.

The core polymer mixture 200 shown in FIG. 2 is an example of athree-phase system. The core polymer mixture 200 is free of colorpigments, UV and thermal stabilizers, process aids and other additivesubstances that are known as such from the art. However, the corepolymer 112 may contain more than three phases, such as e.g. afour-phase system comprising the thread polymer 202, the core polymer112, an additional thread polymer, and the compatibilizer 204. In such afour-phase system, the thread polymer 202 and the additional threadpolymer may be not miscible with the core polymer 112. Thecompatibilizer 204 then separates the thread polymer 202 from the corepolymer 112 and the additional thread polymer from the core polymer 112.In this example the same compatibilizer 204 is used for both the threadpolymer 202 and the additional thread polymer. In other examples, thecompatibilizer 204 used for the thread polymer 202 may be different fromthe compatibilizer 204 used for the additional thread polymer. In afour-phase core polymer mixture 200, the polymer beads 210 may be formedby both the thread polymer 202 and additional thread polymer.

The compatibilizer 204 may be any one of the following: a maleic acidgrafted on polyethylene or polyamide; a maleic anhydride grafted on freeradical initiated graft copolymer of polyethylene, SEBS, EVA, EPD, orpolypropylene with an unsaturated acid or its anhydride such as maleicacid, glycidyl methacrylate, ricinoloxazoline maleinate; a graftcopolymer of SEBS with glycidyl methacrylate, a graft copolymer of EVAwith mercaptoacetic acid and maleic anhydride; a graft copolymer of EPDMwith maleic anhydride; a graft copolymer of polypropylene with maleicanhydride; a polyolefin-graft-polyamidepolyethylene or polyamide; and apolyacrylic acid type compatibilizer. As a consequence of itsinterfacing topology, the compatibilizer 204 may be a major portion (upto 60% by weight) of the core polymer mixture 200 in order to enable afull encasement of the threadlike regions 400 after stretching.

Notwithstanding the above, it is emphasized that the liquid core polymermixture 200, and equivalently, the core 110 formed from the liquid corepolymer mixture 200 during manufacturing, is at least a two-phase systemcomprising the thread polymer 202 as a first one of the at least twophases, and the liquid core polymer 112 as a second one of the at leasttwo phases. This includes the example that the liquid core polymermixture, or respectively, the core, is a two-phase system, i.e., thecore is free of the compatibilizer.

The thread polymer 202 and the core polymer 112 are two chemicallydifferent polymers. In any case, each of the core polymer 112 and thethread polymer form a phase, i.e. a macroscopic, continuous volumefilled with a plurality of molecules of the respective polymer.Consequentially, any beads 210 or threadlike regions 400 formed from thethread polymer 202 are macroscopic phases embedded in the macroscopiccore polymer phase 112. More precisely, any one of the threadlikeregions 400 is not to be understood as a single stretched polymermolecule.

The cladding polymer component is prepared by mixing the pure claddingpolymer granulate with additives as desired for the resulting artificialturf fibers. Suitable additives may be one or more of a wax, a dullingagent, a UV stabilizer, a flame retardant, including aramid fibersand/or an intumescent additive, an anti-oxidant, a fungicide, anantimicrobial agent, such as a silver salt, and/or a pigment, includingan infrared-(IR-) reflective pigment or combinations thereof.

The core polymer mixture 200 and the cladding polymer component are thenmelted in two single-component extrusion units and fed to a coextrusionhead or die, a spinneret, or a similar coextrusion device. The melttemperature used during extrusion is dependent on the types of polymerand compatibilizer 204 that are used.

The melt temperature is typically between 230° C. and 280° C. Apreferable choice of process parameters for the combination of polyamidebeing the thread polymer and polyethylene being both the core polymerand the cladding polymer, is a pressure of 80 bar and a temperature of240° C.

The coextrusion includes joining the cladding polymer component to thecore polymer mixture 200 such that they form a polymer strand of twocomponents which are connected by a contact layer 114 comprising amixture of the core polymer 112 and the cladding polymer. The joiningprocess makes use of a controllable small-scale turbulence to avoidpurely laminar surface-to-surface joining. The control of thisturbulence involves process parameters such as temperature and/or feedrates to influence the rheological behavior (including e.g. viscosity,melt flow index, flow velocity profile) of the two components to bejoined. The strand can be extruded through an extrusion opening to forma bicomponent polymer monofilament 300 of a desired contour. Preferably,the monofilament 300 is quenched after extrusion to fix its structurethus formed.

FIG. 3 shows a cross-section of a small segment of a quenchedmonofilament 300 before stretching. The monofilament 300 is again shownas comprising the core polymer 112 with the polymer beads 210 mixed inand the cladding polymer surrounding the core polymer 112. The polymerbeads 210 are separated from the core polymer 112 by compatibilizer 204which is not shown. To form the threadlike regions, a section of themonofilament 300 is heated and then stretched along an axial directionof the monofilament 300. This is illustrated by the arrows which showthe direction of stretching 310.

FIG. 4 illustrates the effect of stretching the monofilament 300 with anexample of a cross-section of a stretched monofilament 100. The polymerbeads 210 in FIG. 3 have been stretched into threadlike regions. Theamount of deformation of the polymer beads 210 would be dependent uponhow much the monofilament 300 has been stretched.

The polymer beads 210 may comprise crystalline portions and amorphousportions. Stretching the polymer beads 210 into threadlike regions maycause an increase in the size of the crystalline portions relative tothe amorphous portions.

Core 110 and cladding 102 are joined together by a contact layer 114where the core polymer 112 and the cladding polymer are mixed. As can beseen in FIG. 5, the threadlike regions comprised by the core 110 maylocally extend into the contact layer 114 as a consequence of turbulentmixing during joining and of stretching. Preferably, the thread polymer202 amounts to not more than 30% by weight of the core, such that thecohesion provided by the contact layer 114 remains equal or strongerthan in conventional three-component artificial turf fibers with acompatibilizing layer interfacing core and cladding, even if threadpolymer 202 and cladding polymer are not miscible with each other. Thecontact layer 114 may extend radially up to 50 percent of the minimumthickness of the cladding 102 in all directions extending radially fromthe core 110.

FIG. 6 shows a schematic cross-section of an exemplary piece ofartificial turf 600. The artificial turf 600 comprises an artificialturf backing or carpet 602. Artificial turf fiber 604 has been tuftedinto the artificial turf backing 602 to form a pile 608. On the bottomof the artificial turf backing 602 a coating 606 is shown. The coatingmay serve to bind or secure the artificial turf fiber 604 to theartificial turf backing 602. The coating 606 may be optional. Forexample, the artificial turf fibers 604 may be alternatively woven intothe artificial turf backing 602. Various types of glues, coatings oradhesives could be used for the coating 606. The artificial turf fibers604 are shown as forming the pile 608 by extending a distance 610 abovethe artificial turf backing 602. The distance 610 is essentially theheight of the pile 608 of the artificial turf fibers 604. The length ofthe threadlike regions within the artificial turf fibers 604 ispreferably half of the distance 610 or less.

Providing the artificial turf fiber 604 may comprise weaving, spinning,twisting, rewinding, and/or bundling one or more of the stretchedmonofilament 100 into the artificial turf fiber 604. The incorporatingmay comprise weaving or tufting the artificial turf fiber 604 into theartificial turf backing 602.

An effect of designing the protrusions with a slight concave curvaturecan be demonstrated by comparison of FIGS. 7 and 8. FIG. 7 shows anormal cross-sectional profile of an undulated artificial turf fibercomprising a round bulge 700 at the center and two protrusions withrounded tips. The profile extends over an overall thickness t betweenthe front central bulge 700 and the rear tip of the protrusions. Thedistance between the two tips is the overall width w of the fiber. Bothprotrusions have a profile with one straight side 704 and, opposite tothe straight side 704, one undulated side 702 with four notches along astraight base line. Taking into account the axial extension of thefiber, this profile corresponds to protrusions with one flat face andone grooved face.

The protrusions may include an angle between 100 and 180 degrees. In thenon-limiting example shown, the protrusions enclose an angle of about135 degrees towards the undulated side 702 of the profile. Bothprotrusions have a radial extension of about three times the thicknessof the bulge 700. For the purpose of demonstration only, assuming anexemplary overall profile width w=1.35 mm and overall thickness t=0.45mm, the profile of FIG. 7 would have a cross-sectional area of 0.216mm². At an exemplary average density of 0.92 g/mm², this corresponds toa yarn weight of about 2000 dtex.

FIG. 8 shows a normal cross-sectional profile of an undulated artificialturf fiber similar to the one shown in FIG. 7, the difference being thatthe straight sides 704 of the profile are replaced by concave sides 804,corresponding to protrusions with one concave face and one grooved face.The curvature has been designed such that the thickness of theprotrusions (measured between the concave side 804 and the base line ofthe undulated side 702) is gradually declining towards their respectivetip. For comparison with the non-limiting example above, with an overallwidth w=1.35 mm and overall thickness t=0.45 mm as above, the profile ofFIG. 8 would have a cross-sectional area of 0.180 mm². At the assumedaverage density of 0.92 g/mm², this corresponds to a yarn weight ofabout 1650 dtex. A fiber with the concave profile of FIG. 8 would thushave a weight reduction of about 17% compared to a fiber with thestraight profile of FIG. 7. As the concave profile has a slightly largerperimeter than the straight profile, a fiber with the concave profilewould also have an increased surface-to-mass ratio compared to a fiberwith the straight profile.

FIG. 9 illustrates coextrusion of two polymer components in acoextrusion device with a joining path 910 located upstream of acoextrusion opening 908. The setup comprises a hole 906 which receives afree end of a capillary tube 905. Opposite to the inserted capillarytube 905, hole 906 ends in coextrusion opening 908. A clearance betweencapillary tube 905 and the walls of hole 906 hydraulically connects thehole to a second channel system 904. Capillary tube 905 is hydraulicallyconnected to a first channel system 902 and is not fully inserted intohole 906, such that a section 910 of hole 906 is hydraulically connectedboth to first channel system 902 and to second channel system 904. Thissection 910 is the joining path 910 of the depicted coextrusion setup.Joining path 910 extends from capillary tube 905 to extrusion opening908, as is indicated by dotted lines.

During coextrusion operation, capillary tube 905 receives a molten corepolymer component from first channel system 902 and hole 906 receives amolten cladding polymer component from second channel system 904. Therespective transport directions of the polymer components are indicatedby arrows. The two polymer components flow separated from each otheruntil they come into contact in joining path 910. The two joined polymercomponents pass joining path 910, which narrows to the cross section ofcoextrusion opening 908, and exit coextrusion opening 908 as abicomponent monofilament.

In cases where core and cladding are to be joined together with acontact layer comprising a mixture of the core polymer mixture and thecladding polymer component, the dimensions of the joining path aresuitably chosen such that a stable contact layer of homogeneousthickness is formed. In an example, the contact layer is formed withinan axial length of the joining path of 3 to 7 times the diameter of theliquid core polymer mixture at the upstream end of the joining path. Ina more specific example, the diameter of the liquid core polymer mixtureat the upstream end of the joining path is between 0.5 and 1.5 mm, theaxial length of the joining path is between 1.5 and 10.5 mm, causing themelted core polymer mixture to mix with the cladding polymer componentin a contact layer with a radial thickness between 10 and 150 μm.

LIST OF REFERENCE NUMERALS

-   100 stretched monofilament-   102 cladding-   104 protrusion-   110 core-   112 core polymer-   114 contact layer-   200 core polymer mixture-   202 thread polymer-   204 compatibilizer-   210 polymer beads-   300 raw monofilament-   310 direction of stretching-   400 threadlike regions-   600 artificial turf-   602 artificial turf backing-   604 artificial turf fiber-   606 coating-   608 pile-   610 height of pile-   700 central bulge-   702 undulated side-   704 straight side-   804 concave side-   902 first channel system-   904 second channel system-   905 capillary tube-   906 hole-   908 coextrusion opening-   910 joining path section

1. A method for producing an artificial turf fiber, the methodcomprising: preparing a liquid core polymer mixture, the core polymermixture comprising a core polymer and a thread polymer forming beadswithin the core polymer; coextruding the liquid core polymer mixturewith a liquid cladding polymer component into a monofilament, the liquidcore polymer mixture forming a cylindrical core, the liquid claddingpolymer component forming a cladding encompassing the core, the claddinghaving a non-circular profile; quenching the monofilament; reheating thequenched monofilament; stretching the reheated monofilament to deformthe beads into threadlike regions; and providing one or more of thestretched monofilaments as the artificial turf fiber.
 2. The method ofclaim 1, the coextruding further comprising forming the cladding withtwo protrusions which extend from the core in opposite directions. 3.The method of claim 2, the profile of at least one of the protrusionscomprising a concave side.
 4. The method of claim 2, the profile of atleast one of the protrusions comprising an undulated section spanning atleast 60% of one side of said at least one protrusion.
 5. The method ofclaim 1, the coextruding further comprising bringing the liquid corepolymer mixture and the liquid cladding polymer component into contactwith each other such that a contact layer is formed between the liquidcore polymer mixture and the liquid cladding polymer component, thecontact layer comprising a mixture of the liquid core polymer mixtureand the liquid cladding polymer component.
 6. The method of claim 5, thecontacting comprising pressing the liquid core polymer mixture and theliquid cladding polymer component concentrically along a joining path,the liquid core polymer mixture and the liquid cladding polymercomponent being allowed to mix along the joining path to form thecontact layer, the contact layer being formed within an axial length ofthe joining path of 3 to 7 times the diameter of the liquid core polymermixture at the upstream end of the joining path.
 7. The method of claim6, the coextruding being performed such that the liquid core polymermixture and the liquid cladding polymer component enter the joining pathwith unequal flow rates.
 8. The method of claim 6, the diameter of theliquid core polymer mixture at the upstream end of the joining pathbeing between 0.5 and 1.5 mm, preferably 1.25 mm.
 9. The method of claim1, the thread polymer being any one of polyamide, polyethyleneterephthalate, polybutylene terephthalate, polyester, and polybutyrateadipate terephthalate; and/or the core polymer and/or the claddingpolymer being any one of polyethylene, polypropylene, and a mixturethereof.
 10. The method of claim 1, further comprising forming the corewith a diameter of 50 to 600 micrometers, forming the cladding with aminimum thickness of 25 to 300 micrometers in all directions extendingradially from the core, and forming each of the protrusions with aradial extension, measured from the perimeter of the core, of thethickness of the cladding plus 2 to 10 times the radius of the core. 11.The method of claim 1, being performed such that the threadlike regionsassume a diameter of less than 50 μm and/or a length of less than 2 mm.12. The method of claim 1, the core polymer mixture being prepared freeof at least one of the following components of the cladding: a wax, adulling agent, a UV stabilizer, a flame retardant, an anti-oxidant, afungicide, a pigment, and combinations thereof.
 13. The method of claim1, the core polymer being high-density polyethylene, HDPE, and thecladding polymer being linear low-density polyethylene, LLDPE.
 14. Themethod of claim 1, the liquid core polymer mixture being at least athree-phase system, the thread polymer being immiscible with the corepolymer, the core polymer mixture further comprising a compatibilizer,the preparing of the liquid core polymer mixture resulting in the beadsbeing surrounded by the compatibilizer and immersed in the core polymer.15. The method of claim 14, the thread polymer being immiscible with thecladding polymer, the coextruding further comprising bringing the liquidcore polymer mixture and the liquid cladding polymer component intocontact with each other such that a contact layer is formed between theliquid core polymer mixture and the liquid cladding polymer component,the contact layer comprising a mixture of the core polymer and thecladding polymer, the contact layer locally further comprising thecompatibilizer as a third component of the mixture.
 16. The method ofclaim 1, wherein: the core polymer mixture comprises the thread polymerand the additional thread polymer combined in an amount of 1 to 30percent by weight of the core polymer mixture; and/or the core polymermixture comprises the compatibilizer in an amount of 0 to 60 percent byweight of the core polymer mixture; and/or the monofilament comprisesthe cladding polymer in an amount of 50-80 percent by weight of themonofilament.
 17. The method of claim 1, the preparation of the liquidcore polymer mixture comprising: forming a base polymer mixture bymixing the thread polymer, with the compatibilizer; heating the basepolymer mixture; extruding the base polymer mixture; granulating theextruded base polymer mixture; mixing the granulated base polymermixture with the core polymer; and heating the granulated base polymermixture with the core polymer to form the liquid core polymer mixture.18. (canceled)
 19. The method of claim 1, the coextrusion beingperformed at working temperatures between 180 and 270° C.
 20. The methodof claim 1, the liquid core polymer mixture being at least a two-phasesystem comprising the thread polymer as a first one of the at least twophases and the liquid core polymer as a second one of the at least twophases, each of the at least two phases comprising a plurality ofmolecules of the respective polymer.
 21. A method for producing anartificial turf, the method comprising: generating an artificial turffiber by performing the method for producing an artificial turf fiberaccording to any of the previous claims, incorporating the artificialturf fiber into an artificial turf backing, and cutting the artificialturf fiber into sections such that cut surfaces are created which exposethe contact layer.